1. Field of the Invention
The present invention relates to photographic color couplers. In greater detail, it relates to novel photographic 2-equivalent cyan color forming couplers.
2. Description of the Prior Art
It is well known that oxidized aromatic primary amine developing agents react with dye forming couplers to form color images by color development after exposure of silver halide photographic sensitive materials to light. In this method, a color reproduction process according to a conventional subtractive process is utilized to form cyan, magenta and yellow color images which have a complementary relation with red, green and blue colors. For example, phenol derivatives or naphthol derivatives are used as couplers for formation of a cyan color image.
In the color photographic process, the color forming couplers which are added to a developer or incorporated in photosensitive emulsion layers or other color forming layers react with an oxidation product of the color developing agent formed during development to form color images. Hitherto, cyan color forming couplers are usually 4-equivalent couplers, that is, in theory they stoichiometrically require 4 mols of exposed silver halide as an oxidizing agent for formation of 1 mol of the dye. In using 4-equivalent couplers, it is necessary to incorporate a large amount of silver halide in the photosensitive layers. However, incorporation of such large amount of silver halide in the photosensitive layers causes the disadvantages that the sharpness of formed images is deteriorated because of the increase of the scattering of light in the emulsion layers and that the rate of processing the photosensitive materials is reduced because of the increased thickness of the emulsion layer. Further, in using such couplers, a strong oxidizing agent is required in the subsequent processing steps, because the formation of the dyes is not completed in the color development bath.
In order to overcome these defects, use of 2-equivalent couplers has been proposed, that is, couplers which in theory stoichiometrically require only two mols of exposed silver halide for forming 1 mol of indoaniline or indophenol dyes.
2-Equivalent couplers have the structure in which a hydrogen atom of the coupling position, that is, the p-position to a phenolic hydroxyl group is substituted with a releasable group. Examples of such a releasable group are sulfonamido groups as described in U.S. Pat. No. 3,737,316, imido groups as described in U.S. Pat. No. 3,749,735, sulfonyl groups as described in U.S. Pat. No. 3,622,328, aryloxy groups as described in U.S. Pat. No. 3,476,563 and a cyanoacetamido group as described in U.S. Pat. No. 3,808,945.
However, these couplers are not always satisfactory, because they have defects that their coupling reactivity is insufficient, that their dispersibility is poor, thereby giving rise to coating difficulties, that their synthesis is very difficult and that the storability of the formed color images is poor, etc.
Referring particularly to U.S. Pat. No. 3,808,945, which discloses a 2-equivalent naphthol type coupler having a cyanoacetamido group as a releasable group, the coupler provides an extremely poor maximum color density, as will be shown in Example 1 hereinafter, as compared to the coupler of the present invention due to the cyanoacetamido group of the prior art being derived from a carboxylic acid having a pKa outside the critical range set forth in the present invention. Further, only from similarity in structure of releasable groups, U.S. Pat. No. 3,291,609 may be considered to be the prior art since it discloses phenol type compounds having amido groups. The phenol type compounds taught in U.S. Pat. No. 3,291,609 are, however, distinguishable over the coupler of the present invention because the phenol type compounds are employed as developing agent precursors, whereas the coupler of the present invention does not act as a developing agent. While it lacks a definite proof, this is believed due to difference in reduction-oxidation balance required in acting as a developing agent.